Among wild animals living in liberty hybrids are still more rare. It is unknown, for example, among mammalia, according to Isidore Geoffroy, whose experience has here a double value. The order of birds alone presents some facts of this kind, nearly all of which are in the order of Gallinæ. According to Valenciennes, they are unknown among fishes. In domestication and captivity spontaneous crossing between different species is a little less rare. The intelligent intervention of man has multiplied unions of this kind in a remarkable manner, especially among plants, but without being able to extend their limits. Linnæus thought crossing was possible between species of _different families_. But in 1761 Koelreuter showed that he was mistaken. From these investigations, which were carried on for twenty-seven years, and from those of M. Naudin, his worthy rival, it appears that artificial crossing between species of _different families never_ succeeds, and _very rarely_ between species of _different genera_; that it is always very difficult, and demands the most minute precautions to insure success; that it often fails between species of the same genus closely allied in appearance, and finally, that there are whole families among which hybrids are impossible. Amongst the latter figures the family of the cucurbitaceæ, so thoroughly studied by M. Naudin, where the most perfect mongrels were produced spontaneously. We could not imagine, evidently, a more complete contrast. This contrast is carried into the minutest details. For example, any flower which has in the least possible degree undergone the action of pollen of its own species becomes absolutely insensible to the action of pollen of a different species. How different to the equality of action displayed by the several pollens of most distant races! All experimenters agree further in declaring that even in the unions between species which have been most successful, the fertility is constantly diminished, and often in immense proportions. The head of the Papaver somnifera generally contains 2000 seeds or more. In a hybrid of this species Gœrtner only found six which had been matured; all the rest were more or less abortive. Here again, what a contrast between the crossing productive of such fertility in M. De Ginestous’ English pigs.